In designing electronic circuitry, an interface circuit is frequently used to provide an output signal representative of the open or closed state of a remotely-located set of switch contacts while maintaining electrical isolation between the switch contacts and the output signal. In burner control systems, for example, a large number of switches monitor various conditions in the burner installation. The states of these switches must be provided to the burner control circuitry while maintaining electrical isolation between the switches and the control circuitry, to prevent possible damage to the control circuitry. In addition to providing isolation, the interface circuit must provide a high degree of noise immunity. Especially in field situations, switch contacts are frequently contaminated by dirt and may go long periods between maintenance. Such operational conditions impose stringent requirements on the interface circuitry.
To test the open or closed state of a set of field located contacts, a moderately high current may be passed through these contacts. Contamination of the contacts generally results in a high resistance shunt around the contacts, and this contamination cannot pass the high current levels necessary to provide an indication of a closed contact state. Additionally, the high current level provides an increased noise immunity to spurious signals capacitively coupled to the contacts or interconnecting cabling.
Although several techniques are known in the prior art for providing such interface circuits, these techniques have several drawbacks. Relays may be used to provide isolation between circuits connected to the relay coil and contacts. However, the cost and size of relays makes them uneconomical for situations in which a large number of contacts must be monitored. Additionally, relays have a finite contact life and require periodic maintenance. Optical isolators or photocouplers are frequently used to detect the state of switch contacts. Currently available optical isolator circuits are semiconductor devices whose parameters are extremely variable. The gain through an optical isolator may vary over a 10:1 ratio, for example. This results in more complicated circuit designs and frequently requires some sort of in-circuit adjustment. Additionally, the reliability of semiconductor devices declines when they are exposed to certain extreme environmental conditions of heat or high voltages.